Brushless motors commonly include permanent rotor magnets with one or more pairs of poles having opposite magnetic polarity. In such motor configurations, the magnetic attraction between the rotor poles and the stator poles changes as the rotor rotates with respect to the stator. This variance in magnetic attraction as a function of rotor position is known as cogging. Several problems are caused by cogging, as discussed in the various patents above. Among these are a variable torque characteristic while the rotor is rotating, and the existence of preferred relative positions between the rotor and stator. In the first case, cogging results in decreased operational or steady-state efficiency. In the latter case, cogging reduces the ability to stop the motor in a desired position because the rotor tends to align itself into one of a limited number of positions relative to the stator poles. The ability to stop a motor at precise positions may be critical in certain control applications. In such situations where the desired rotational stopping position is not at a point of polar alignment, a motor subject to cogging effects may be inadequate or less desirable. Cogging results in torque and speed variations in permanent magnet electric motors due to the magnetic flux variations as the rotor poles move past the stator poles. In this regard, the cogging appears as a variable AC torque component. The reluctance of the motor air gap is significantly higher at the stator slots than at the pole teeth, thus causing cogging. A reduction in the reluctance variation will thus result in a reduction in cogging torque and its associated problems.
In a typical permanent magnet rotor, a single piece cylindrical magnet is mounted on a cylindrical shaft and charged to include at least one pair of poles having opposite polarity. This configuration, while economical to produce, suffers from high cogging torque. Heretofore, several different attempts have been made to reduce permanent magnet electric motor cogging effects, as discussed in the foregoing patents incorporated by reference. One method includes skewing the stator winding slots with respect to the permanent magnet pole edges, as shown in Musil U.S. Pat. No. 4,424,463. Another method involves skewing the permanent magnet pole pieces with respect to the stator winding slots. Cogging torque may also be reduced by providing a motor where the number of stator salient poles is less than the number of rotor permanent magnet poles as shown in Kawasaki, et al. U.S. Pat. No. 3,860,843. These practices, while reducing cogging effects, greatly complicate the manufacturing process, and correspondingly increase manufacturing costs. Another approach is shown in Sievert U.S. Pat. No. 4,341,969, wherein the edges of the permanent magnet pole pieces are cut or machined to form a series of notches on the leading and trailing edges of the stator poles. Similarly, in Mulgrave U.S. Pat. No. 5,783,890, the leading and trailing edges of the rotor permanent magnets are magnetized in a longitudinally varying magnetization strength pattern, while the central portion of each magnet is uniformly magnetized. Selective magnetization, like selective machining or forming of permanent magnets, increases the cost of manufacturing. Still another attempt at reducing cogging torque is where a rotor includes a square shaft and four section-shaped magnet pieces are attached to the flat outer surfaces of the square shaft. As discussed further hereinafter, this method significantly increases the manufacturing cost of permanent magnet rotors due to the specialized shape of the shaft and the use of separate magnet pieces. Consequently, there remains a need for an improved permanent magnet rotor which reduces cogging and adds little or no manufacturing costs.